[Insight-users] iteration number and optimizer values

[Michael Kuhn]

Hi Luis,

I've again had a look at the values obtained by my observer (an 
itk::Command object) that is observing a 
QuaternionRigidTransformGradientDescentOptimizer. I believe to have 
noticed the follwing things:

1. ) At iteration n, the metric value does not correspond to the current 
position. The event is only triggered after the new position is set to 
the optimizer (in QuaternionRigidTransformGradientDescentOptimizer):

  this->SetCurrentPosition( newPosition );
  this->InvokeEvent( IterationEvent() );

Therefore, the optimizer already returns the position based on which the 
metric value is calculated in iteration n + 1. Thus, the position 
obtained from the optimizer by calling optimizer->GetCurrentPosition() 
in iteration n corresponds to the metric value obtained by calling 
optimizer->GetValue() in iteration n + 1. (compare the "pseudo output" 
below)


2. ) The iteration number obtained by optimizer->GetCurrentIteration() 
(say, n) is increased by 1 compared to the iteration number of the last 
iteration event (n - 1). The transform is the "new position" set before 
invoking the last iteration event (n - 1) and would thus corresponds to 
the metric value in iteration n (that is not executed, since we are at 
the end of the optimization process). However, there is no new metric 
value computed, therefore the metric value is the same as the one 
obtained during the last iteration event (n - 1). Therefore, the metric 
value now differs by two steps from the position. (compare the "pseudo 
output" below)

I think, for debugging purposes it would be helpful if the three values 
would correspond with respect to their index in every event.

Thanks,

Michael


pseudo output of an observer:
====================
---typeid(event): start--------
current iteration 0
current position: p0 (= initial guess)
metric value: 0
------------------------------

---typeid(event): iteration-----
current iteration: 0
current position p1
metric value: m0
------------------------------

---typeid(event): iteration-----
current iteration: 1
current position p2
metric value: m1
------------------------------

---typeid(event): iteration-----
current iteration: 2
current position p3
metric value: m2
------------------------------

---typeid(event):end---------
current iteration: 3
current position p4
metric value: m2
------------------------------




<http://www . itk . org/Insight/Doxygen/html/classitk_1_1QuaternionRigidTransformGradientDescentOptimizer . html>